Caprock compressibility and permeability and the consequences for pressure development in CO₂ storage sites

Large scale CO₂ storage has previously been considered for the Vedsted structure located in the Northern part of Jylland in Denmark. Pressure buildup in the Gassum reservoir and transmission to the shallower Chalk Group where the brine-fresh water interface resides need to be investigated as part of site qualification, as overpressure can push brine into the fresh water zone and thereby affecting aquifer performance. Pressure transmission from the reservoir into the surrounding formations, when fractures and faults are ignored, will depend on the properties and thickness of the sealing rock. The most important property to be considered is caprock compressibility and permeability. Laboratory experiments on centimeter-scale plugs and dynamic sonic velocity data from relevant shale formations in Denmark indicate that shale compressibility is lower than often assumed for reservoir simulation studies. The measured compressibility for the Fjerritslev Formation is 0.5×10 ^-5bar ^-1, which is an order of magnitude lower than the standard compressibility (4.5×10 ^-5bar ^-1) normally used for reservoir simulation studies. The consequences of this lower compressibility are investigated in a simulation case study and the results indicate that higher overpressure is created in the reservoir and the caprock. Overestimating caprock compressibility can therefore underestimate overpressure within the storage and sealing formations and this can have significant implication in the presence of highly permeable fractures and faults. The caprock permeability is measured on core samples using a geotechnical method of constant rate of strain (CRS) experiments which seem to match the modeled permeability data for the Fjerritslev Formation. We found an average vertical permeability of 0.1μD for the Fjerritslev Formation from the samples measured. The sensitivity of pressure development for the caprock permeability has been studied by varying from one to three orders of magnitude higher and one to two orders of magnitude lower than the measured permeability of 0.1μD. Injecting 60 million tons (Mt) of CO ₂ at a rate of 1.5Mt/year into the Gassum Formation for 40 years indicates that, with permeability above 1.0μD, overpressure can be transmitted through the 530m thick Fjerritslev Formation caprock and further up into the overburden layers.